On taking a cell phone apart, one would find a few individual parts, such as, an antenna, a liquid crystal display (LCD), a keyboard, a microphone, a speaker, and a battery. Inside the phone there is a circuit board which is otherwise called the heart of the system. First you would enter in room analog-to-digital and digital-to-analog conversion chips which translate the outgoing audio signal from analog to digital and the incoming signal from digital back to analog. It can process millions of calculations per second in order to compress and decompress the voice stream. Then you would enter into the second room where the digital signal processor (DSP) is located.
Here in this room you would soon discover that the DSP is a highly customized processor designed to perform signal manipulation calculations at high speed. Next, you would enter the third room with the microprocessor. The microprocessor handles all the main functions for the keyboard and display, deals with command and control signaling with the base station (cell tower) and also coordinates the rest of the functions on the board. The read only memory (ROM) and Flash memory chips provide storage for the phone’s operating system and customization features, such as the phone directory.
The radio frequency (RF) and power section handles power management and recharging, and also deals with the hundreds of FM channel. Finally, the RF amplifiers handle signals traveling to and from the antenna.
An analog signal has a base carrier’s radio frequency signal, which is modified in some way to amplify the strength of the signal or vary the frequency to add information to the signal. An analog signal can be represented as a series signal to a signal carrier known as sine waves because carrier waves are analogous to the fluctuations of the human voice or other sound that is being transmitted. Advanced Mobile Phone System (AMPS), the first common cell phone system in the U.S., uses a range of frequencies between 824MHz and 894 MHz for analog cell phones. A Hertz (Hz) is equal to cycles per second, while a MegeHertz (MHz) is equal to one million cycles per second.
The frequencies chosen to be used in analog voice channels are 30 kHz wide, because it gives voice quality that is comparable to a wired telephone. The transmitter and receiver frequencies of each voice channel are separated by 45 MHZ, to keep them from interfering with each other. Each carrier has 395 voice channels and 21 control channels for activities of registration and paging. Each cell only uses about one-seventh of its frequencies. This helps a hexagonal cell and the six cells in the grid to all use the frequencies. Digital cell phones use the same radio technology in a different way.
For instance, digital phones change voice into binary information (1 and 0) and then compresses it where this compression allows ten digital phones to occupy the same frequency space as one analog cell phone.
Speech is sound in motion, but talking produces acoustic pressure. A telephone reproduces sound by electrical means. However, in wireless technology, a coder inside the mobile telephone converts sound to digital impulses on the transmitting side and on the receiving side it converts these impulses back to analog sounds.
A coder or vocoder is a speech analyzer and synthesizer all in one. A vocoder is found in every digital wireless telephone and is part of a larger chip set called a digital signal processor. In this process, sound gets modeled and transmitted on one end of the vocoder and on the receiving end, the speech synthesizer part, interprets the signal and produces a close match of the original.
These sounds can vary because of telephone circuit’s resistance, while electrically representing speech with a continuous (analog) electromagnetic wave; however, digital signals remain stable for the length of their travel because digital signals are a mathematical or numerical representation of sound, with each sonic nuance captured as a binary number.
On hearing sounds, your ears are responding to tiny, rapid changes in the pressure of the air. These changes are called sound waves. They can have a single frequency and constant amplitude. Hearing is a complex mixture of waves with different frequencies and amplitudes. Sound waves range from pure sine waves to complex combinations of waves. The normal human ear can perceive sound ranging in frequency from 20 to 20,000 Hz.16